Context. The astrochemistry of the important biogenic element phosphorus (P) is still poorly understood, but observational evidence indicates that P-bearing molecules are likely associated with ...shocks. Aims. We study P-bearing molecules and some shock tracers towards one of the chemically richest hot molecular cores, G31.41+0.31, in the framework of the project “G31.41+0.31 Unbiased ALMA sPectral Observational Survey” (GUAPOS), which is being carried out with the Atacama Large Millimeter Array (ALMA). Methods. We observed the molecules PN, PO, SO, SO2, SiO, and SiS through their rotational lines in the spectral range 84.05– 115.91 GHz covered by the GUAPOS project. Results. PN is clearly detected, while PO is tentatively detected. The PN emission arises from two regions southwest of the hot core peak, named regions 1 and 2 here, and is undetected or tentatively detected towards the hot core peak. The PN and SiO lines are very similar both in spatial emission morphology and spectral shape. Region 1 is partly overlapping with the hot core and is warmer than region 2, which is well separated from the hot core and located along the outflows identified in previous studies. The SO, SO2, and SiS emissions are also detected towards the PN-emitting regions 1 and 2, but arise mostly from the hot core. Moreover, the column density ratio SiO/PN remains constant in regions 1 and 2, while SO/PN, SiS/PN, and SO2/PN decrease by about an order of magnitude from region 1 to region 2, indicating that SiO and PN have a common origin even in regions with different physical conditions. The PO/PN ratio in region 2, where PO is tentatively detected, is ~0.6–0.9, which is in line with the predictions of pure shock models. Conclusions. Our study provides robust confirmation of previous observational evidence that PN emission is tightly associated with SiO and is likely a product of shock chemistry, as the lack of a clear detection of PN towards the hot core allows us to rule out relevant formation pathways in hot gas. We propose the PN-emitting region 2 as a new astrophysical laboratory for shock-chemistry studies.
Context. Molecular outflows powered by young protostars strongly affect the kinematics and chemistry of the natal molecular cloud through strong shocks. This results in substantial modifications of ...the abundance of several species. In particular, water is a powerful tracer of shocked material because of its sensitivity to both physical conditions and chemical processes. Aims. As part of the Chemical HErschel Surveys of Star-forming regions (CHESS) guaranteed time key program, we aim at investigating the physical and chemical conditions of H2O in the brightest shock region B1 of the L1157 molecular outflow. Methods. We observed several ortho- and para-H2O transitions using the HIFI and PACS instruments on board Herschel toward L1157-B1, providing a detailed picture of the kinematics and spatial distribution of the gas. We performed a large velocity gradient (LVG) analysis to derive the physical conditions of H2O shocked material, and ultimately obtain its abundance. Results. We detected 13 H2O lines with both instruments probing a wide range of excitation conditions. This is the largest data set of water lines observed in a protostellar shock and it provides both the kinematics and the spatial information of the emitting gas. The PACS maps reveal that H2O traces weak and extended emission associated with the outflow identified also with HIFI in the o-H2O line at 556.9 GHz, and a compact (~10′′) bright, higher excitation region. The LVG analysis of H2O lines in the bow-shock show the presence of two gas components with different excitation conditions: a warm (Tkin ≃ 200–300 K) and dense (n(H2) ≃ (1–3) × 106 cm-3) component with an assumed extent of 10′′, and a compact (~2′′–5′′) and hot, tenuous (Tkin ≃ 900–1400 K, n(H2) ≃ 103−4 cm-3) gas component that is needed to account for the line fluxes of high Eu transitions. The fractional abundance of the warm and hot H2O gas components is estimated to be (0.7–2) × 10-6 and (1–3) × 10-4, respectively. Finally, we identified an additional component in absorption in the HIFI spectra of H2O lines that connect with the ground state level. This absorption probably arises from the photodesorption of icy mantles of a water-enriched layer at the edges of the cloud, driven by the external UV illumination of the interstellar radiation field.
The unbearable opaqueness of Arp220 Martín, S.; Aalto, S.; Sakamoto, K. ...
Astronomy and astrophysics (Berlin),
06/2016, Letnik:
590
Journal Article
Recenzirano
Odprti dostop
Context. The origin of the enormous luminosities of the two opaque nuclei of Arp 220, the prototypical ultra-luminous infrared galaxy, remains a mystery because we lack observational tools to explore ...the innermost regions around the nuclei. Aims. We explore the potential of imaging vibrationally excited molecular emission at high angular resolution to better understand the morphology and physical structure of the dense gas in Arp 220 and to gain insight into the nature of the nuclear powering sources. Methods. The Atacama Large Millimeter/submillimeter Array (ALMA) provided simultaneous observations of HCN, HCO+, and vibrationally excited HCN v2 = 1f emission. Their J = 4–3 and 3–2 transitions were observed at a matching resolution of ~0.5′′, which allows us to isolate the emission from the two nuclei. Results. The HCN and HCO+ lines within the ground-vibrational state poorly describe the central ~100 pc region around the nuclei because there are strong effects of cool absorbing gas in the foreground and severe line blending that is due to the prolific molecular emission of Arp 220. Vibrationally excited emission of HCN is detected in both nuclei with a very high ratio relative to the total LFIR, higher than in any other observed galaxy and well above what is observed in Galactic hot cores. HCN v2 = 1f is observed to be marginally resolved in ~60 × 50 pc regions inside the dusty ~100 pc sized nuclear cores. Its emission is centered on our derived individual nuclear velocities based on HCO+ emission (VWN = 5342 ± 4 and VEN = 5454 ± 8 km s-1, for the western and eastern nucleus, respectively). With virial masses within r ~ 25–30 pc based on the HCN v2 = 1f line widths, we estimate gas surface densities (gas fraction fg = 0.1) of 3 ± 0.3 × 104 M⊙ pc-2 (WN) and 1.1 ± 0.1 × 104 M⊙ pc-2 (EN). The 4−3/3−2 flux density ratio could be consistent with optically thick emission, which would further constrain the size of the emitting region to >15 pc (EN) and >22 pc (WN). The absorption systems that may hide up to 70% of the HCN and HCO+ emission are found at velocities of −50 km s-1 (EN) and 6, −140, and −575 km s-1 (WN) relative to velocities of the nuclei. Blueshifted absorptions are the evidence of outflowing motions from both nuclei. Conclusions. Although vibrationally excited molecular transitions could also be affected by opacity, they may be our best tool to peer into the central few tens of parsecs around compact obscured nuclei like those of Arp 220. The bright vibrational emission implies the existence of a hot dust region radiatively pumping these transitions. We find evidence of a strong temperature gradient that would be responsible for both the HCN v2 pumping and the absorbed profiles from the vibrational ground state as a result of both continuum and self-absorption by cooler foreground gas.
Context.
Phosphorus (P) is a crucial element for life given its central role in several biomolecules. P-bearing molecules have been discovered in different regions of the Milky Way, but not yet ...towards an extragalactic environment.
Aims.
We searched for P-bearing molecules outside the Milky Way towards the nearby starburst Galaxy NGC 253.
Methods.
Using observations from the ALMA Comprehensive High-resolution Extragalactic Molecular Inventory (ALCHEMI) project, we used the MAdrid Data CUBe Analysis package to model the emission of P-bearing molecules assuming local thermodynamic equilibrium (LTE) conditions. We also performed a non-LTE analysis using SpectralRadex.
Results.
We report the detection of a P-bearing molecule, phosphorus nitride (PN), for the first time in an extragalactic environment, towards two giant molecular clouds (GMCs) of NGC 253. The LTE analysis yields total PN beam-averaged column densities
N
= (1.20 ± 0.09) × 10
13
cm
−2
and
N
= (6.5 ± 1.6) × 10
12
cm
−2
, which translate into abundances with respect to H
2
of
χ
= (8.0 ± 1.0) × 10
−12
and
χ
= (4.4 ± 1.2) × 10
−12
. We derived a low excitation temperature of
T
ex
= (4.4 ± 1.3) K towards the GMC with the brightest PN emission, which indicates that PN is sub-thermally excited. The non-LTE analysis results in column densities consistent with the LTE values. We also searched for other P-bearing molecules (PO, PH
3
, CP, and CCP), and upper limits were derived. The derived PO/PN ratios are < 1.3 and < 1.7. The abundance ratio between PN and the shock-tracer SiO derived towards NGC 253 follows the same trend previously found towards Galactic sources.
Conclusions.
Comparison of the observations with chemical models indicates that the derived molecular abundances of PN in NGC 253 can be explained by shock-driven chemistry followed by cosmic-ray-driven photochemistry.
ABSTRACT
Hydrogen cyanide (HCN) and its isomer hydrogen isocyanide (HNC) play an important role in molecular cloud chemistry and the formation of more complex molecules. We investigate here the ...impact of protostellar shocks on the HCN and HNC abundances from high-sensitivity IRAM 30 m observations of the prototypical shock region L1157-B1 and the envelope of the associated Class 0 protostar, as a proxy for the pre-shock gas. The isotopologues H12CN, HN12C, H13CN, HN13C, HC15N, H15NC, DCN, and DNC were all detected towards both regions. Abundances and excitation conditions were obtained from radiative transfer analysis of molecular line emission under the assumption of local thermodynamical equilibrium. In the pre-shock gas, the abundances of the HCN and HNC isotopologues are similar to those encountered in dark clouds, with an HCN/HNC abundance ratio ≈1 for all isotopologues. A strong D-enrichment (D/H ≈ 0.06) is measured in the pre-shock gas. There is no evidence of 15N fractionation neither in the quiescent nor in the shocked gas. At the passage of the shock, the HCN and HNC abundances increase in the gas phase in different manners so that the HCN/HNC relative abundance ratio increases by a factor 20. The gas-grain chemical and shock model uclchem allows us to reproduce the observed trends for a C-type shock with pre-shock density n(H) = $10^5\hbox{cm$^{-3}$}$ and shock velocity $V_\mathrm{ s}= 40\hbox{kms$^{-1}$}$. We conclude that the HCN/HNC variations across the shock are mainly caused by the sputtering of the grain mantle material in relation with the history of the grain ices.
We present observations of SO and SO2 lines toward the shocked regions along the L1157 chemically rich outflow, taken in the context of the Seeds of Life in Space IRAM Northern Extended Millimeter ...Array Large Program, and supported by data from the Submillimeter Array and IRAM-30 m telescope at 1.1-3.6 mm wavelengths. We simultaneously analyze, for the first time, all of the brightest shocks in the blueshifted lobe, namely, B0, B1, and B2. We found the following. (1) SO and SO2 may trace different gas, given that the large(-scale) velocity gradient analysis indicates for SO2 a volume density ( ) denser than that of the gas emitting in SO by a factor up to an order of magnitude. (2) Investigating the 0.1 pc scale field of view, we note a tentative gradient along the path of the precessing jet. More specifically, decreases from the B0-B1 shocks to the older B2. (3) At a linear resolution of 500-1400 au, a tentative spatial displacement between the two emitting molecules is detected, with the SO peak closer (with respect to SO2) to the position where the recent jet is impinging on the B1 cavity wall. Our astrochemical modeling shows that the SO and SO2 abundances evolve on timescales less than about 1000 years. Furthermore, the modeling requires high abundances (2 × 10−6) of both and S/H injected in the gas phase due to the shock occurrence, so prefrozen OCS only is not enough to reproduce our new observations.
The study of hot corinos in solar-like protostars has been so far mostly limited to the Class 0 phase, hampering our understanding of their origin and evolution. In addition, recent evidence suggests ...that planet formation starts already during Class I phase, which therefore represents a crucial step in the future planetary system chemical composition. Hence, the study of hot corinos in Class I protostars has become of paramount importance. Here, we report the discovery of a hot corino towards the prototypical Class I protostar L1551 IRS5, obtained within the ALMA (Atacama Large Millimeter/submillimeter Array) Large Program FAUST (Fifty AU STudy of the chemistry in the disc/envelope system of solar-like protostars). We detected several lines from methanol and its isotopologues (13CH3OH and CH2DOH), methyl formate, and ethanol. Lines are bright towards the north component of the IRS5 binary system, and a possible second hot corino may be associated with the south component. The methanol lines' non-LTE analysis constrains the gas temperature (∼100 K), density (≥1.5 × 10^8 per cu.cm), and emitting size (∼10 au in radius). All CH3OH and 13CH3OH lines are optically thick, preventing a reliable measure of the deuteration. The methyl formate and ethanol relative abundances are compatible with those measured in Class 0 hot corinos. Thus, based on this work, little chemical evolution from Class 0 to I hot corinos occurs.
Glycolaldehyde (HOCH$_2$CHO) and ethylene glycol ((CH$_2$OH)$_2$) are two complex organic molecules detected in the hot cores and hot corinos of several star-forming regions. The ethylene ...glycol/glycolaldehyde abundance ratio seems to show an increase with the source luminosity. In the literature, several surface-chemistry formation mechanisms have been proposed for these two species. With the UCLCHEM chemical code, we explored the different scenarios and compared the predictions for a range of sources of different luminosities with the observations. None of the scenarios reproduce perfectly the trend. A better agreement is, however, found for a formation through recombination of two HCO radicals followed by successive hydrogenations. The reaction between HCO and CH$_2$OH could also contribute to the formation of glycolaldehyde in addition to the hydrogenation pathway. The predictions are improved when a trend of decreasing H$_2$ density within the core region with T $\geq$ 100 K as a function of luminosity, is included in the model. Destruction reactions of complex organic molecules in the gas phase would also need to be investigated, since they can affect the abundance ratios once the species have desorbed in the warm inner regions of the star-forming regions.
Context. Determining the evolution of the CNO isotopes in the interstellar medium (ISM) of starburst galaxies can yield important constraints on the ages of super star clusters (SSCs), or on other ...aspects and factors contributing to their evolution, such as the initial mass function (IMF). Due to the time-dependent nature of the abundances of isotopes within the ISM – as they are supplied from processes such as nucleosynthesis or chemical fractionation –, this provides the opportunity to test whether or not isotope ratios trace the ages of highly star-forming regions, such as SSCs. Aims. The goal of this study is to investigate whether the isotopic variations in SSC regions within NGC 253 are correlated with their different ages as derived from stellar population modelling. Methods. We measured abundance ratios of CO, HCN, and HCO + isotopologues in six regions containing SSCs within NGC 253 using high-spatial-resolution (1.6″, ∼28 pc) data from the ALCHEMI (ALma Comprehensive High-resolution Extragalactic Molecular Inventory) ALMA Large program. We then analysed these ratios using RADEX radiative transfer modelling, with the parameter space sampled using the nested sampling Monte Carlo algorithm MLFriends. These abundance ratios were then compared to ages predicted in each region via the fitting of observed star-formation tracers (such as Br γ ) to Starburst99 starburst stellar population evolution models. Results. We determined the isotopic column density ratios across multiple regions of SSC activity in NGC 253 using non-LTE radiative transfer modelling. We do not find any significant trend with age for the CO and HCN isotopologue ratios on timescales of the ages of the SSC* regions observed. However, HCO + may show a correlation with age over these timescales in 12 C/ 13 C. Conclusions. The driving factors of these ratios within SSCs could be the IMF or fractionation effects. To further probe these effects in SSCs over time, a larger sample of SSCs must be observed spanning a larger age range.
Aims. We study the feedback of star formation and nuclear activity on the chemistry of molecular gas in NGC 1068, a nearby (D = 14 Mpc) Seyfert 2 barred galaxy, by analyzing whether the abundances of ...key molecular species such as ethynyl (C2H), which is a classical tracer of photon dominated regions (PDR), change in the different environments of the disk of the galaxy. Methods. We used the Atacama Large Millimeter Array (ALMA) to map the emission of the hyperfine multiplet of C2H(N = 1−0) and its underlying continuum emission in the central r ≃ 35″ (2.5 kpc) region of the disk of NGC 1068 with a spatial resolution 1.̋0 × 0.̋7 (≃ 50−70 pc). We used maps of the dust continuum emission obtained at 349 GHz by ALMA to derive the H2 gas column densities and combined these with the C2H map at matched spatial resolution to estimate the fractional abundance of this species. We developed a set of time-dependent chemical models, which include shocks, gas-phase PDRs, and gas-grain chemical models to determine the origin of the C2H gas. Results. A sizeable fraction of the total C2H line emission is detected from the r ≃ 1.3 kpc starburst (SB) ring, which is a region that concentrates the bulk of the recent massive star formation in the disk traced by the Paα emission complexes imaged by the Hubble Space Telescope (HST). However, the brightest C2H emission originates from a r ≃ 200 pc off-centered circumnuclear disk (CND), where evidence of a molecular outflow has been previously found in other molecular tracers imaged by ALMA. We also detect significant emission that connects the CND with the outer disk in a region that probes the interface between the molecular disk and ionized gas outflow out to r ≃ 400 pc. We derived the fractional abundances of C2H (X(C2H)) assuming local thermodynamic equilibrium (LTE) conditions and a set of excitation temperatures (Tex) constrained by the previous multiline CO studies of the galaxy. Our estimates range from X(C2H) ≃ a few 10-8 in the SB ring up to X(C2H) ≃ a few 10-7 in the outflow region. The PDR models that incorporate gas-grain chemistry are able to account for X(C2H) in the SB ring for moderately dense (n(H2) ≥ 104 cm-3) and moderately UV-irradiated gas (UV-field ≤ 10 × Draine field, where 1 Draine field ≡ 2.74 × 10-3 erg s-1 cm-2) in a steady-state regime, which depending on the initial and physical conditions of the gas may be achieved by 105 yr or as late as 107 yr. However, the high fractional abundances estimated for C2H in the outflow region can only be reached at very early times (T ≤ 102−3 yr) in models of UV or X-ray irradiated dense gas (n(H2) ≥ 104−5 cm-3). Conclusions. We find that the transient conditions required to fit the high values of X(C2H) in the outflow are likely due to UV or X-ray irradiated non-dissociative shocks associated with the highly turbulent interface between the outflow and molecular gas in NGC 1068. Although the inferred local timescales are short, the erosion of molecular clouds by the active galactic nucleus (AGN) wind and/or the jet likely resupplies the interface working surface continuously, making a nearly steady state persist in the disk of the galaxy.
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